Friday, February 7, 2014

Layering Waves at Darwin C

A fresh impact crater that poured ejecta down the wall of Darwin C in a 7 km field of view from LROC NAC mosaic M1145254989RL, spacecraft orbit 20807, January 25, 2014, resolution 0.8 meters, north toward left [NASA/GSFC/Arizona State University].

Raquel Nuno
LROC Featured Image

We previously published a portion of the spectacular ejecta pattern of an unnamed fresh impact crater: "Impact Art," November 21, 2013.

Today's Featured Image is a broader LROC NAC mosaic of the source where that ejecta originated. This unnamed crater does not have a circular shape, because it formed on the sloped wall of Darwin C crater (15.3 km, 20.51°S, 71.12°W).

Soon after impact, gravity took over, pulling large boulders and fine debris down the crater wall. Zooming in on the ejecta, you can see roughly boulders roughly 6 meters in size that "etched" their way down the crater wall, leaving trails in the granular ejecta as they tumbled downhill.

Boulders rolled down slope leaving evidence of their trek in their wake. They ended their journey after the impact that formed the fresh crater on the wall of crater Darwin C, as can be seen by the overprinting of their trails on the ejecta. The boulders are roughly 6 meters in diameter [NASA/GSFC/Arizona State University].

Since we don't have samples from this crater, we don't know exactly when this fresh impact crater occurred. But, we can assume that it must be relatively young.

The reason why we know it must be young is that exposure to the space environment (usually referred to as space weathering) reduces the albedo of surface materials. This means that scientists can use LROC images to get a relative sense for how long a surface has been exposed to the space environment. Getting some perspective with the wider view, combined with examination of the 643 nm normalized WAC reflectance map which enhances relative reflectance (see image below), you can see the relative brightness difference between the high-reflectance ejecta from our fresh crater compared to crater Darwin C and the rest of the surrounding terrain - this crater is not only superposes Darwin C, but its ejecta is significantly higher reflectance than anything else in the scene.

LROC Wide Angle Camera (WAC) 643 nm normalized reflectance map of Darwin C crater. The bright area is ejecta from the fresh impact crater on the east wall of Darwin C [NASA/GSFC/Arizona State University].

Taken together with the sharp, well-defined rims and lack of other superposed impacts, we can infer that this crater is a relatively recent addition to the lunar surface. The impact event that formed this crater almost certainly occurred within the last several hundred million years, which is practically yesterday over the billions of years of geologic time!

Follow the boulders' trails with the full resolution NAC mosaic, HERE.